Two-way radio communication system



Aug. 15, 1933. R. c MATHES TWO-WAY RADIO COMMUNICATION SYSTEM Filed Nov.1, 1924 ANZ;

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Patented Aug. 15, 1933 TWO-WAY RADIO COMMUNICATION SYSTEM Robert C.Mathes, Wyoming, N. J., assignor to Western Electric Company,Incorporated, New York, N. Y., a Corporation of New York ApplicationNovember 1, 1924. Serial No. 747,164

"9 Claims.

This invention relates to two-way radio communication systems andespecially to four-wire systems constituted by the combination of a landline and a radio link.

One object of the invention is to provide improved methods of andapparatus for operating four-wire circuits. The methods of thisinvention make possible the operation of such systems with a substantialavoidance of echo and singing effects and with markedly greatersimplicity and lower cost, than results from the use of means heretoforeknown.

Another object of the invention is to adapt the principles governing theeflicient operation of a four-wire land line to the operation of afour-wire circuit comprising the combination of a land line and aradiolink, and to realize therein the advantages usually accruing tofour-wire operation.

A four-wire circuit, as defined for present purposes, is a two-waycircuit in the operation oi which the transmission in one direction forthe greater portion of the distance between two points geographicallyremote from each other is carried on over a diii crent path fromtransmission in the other direction. Such a circuit is distinguishedfrom a two-wire circuit the operation of which is characterized by theuse of the same path for transmission in both directions for the greaterportion of the distance between the two points.

In the operation of four-wire and two-wire circuits it is desirable thatthe circuit be operative for transmission in only one direction at atime. The purpose in sodoing is to avoid singa ing and echo effects.

The nature of singing and echo phenomena is well understood as theyexist in fourwire and twc-wire circuits and will not require exposition40 here, reference being made to a paper by A; B.

Clark entitled Telephone transmission over long cables (Journal of theA. I. E. E., January 1923) whereinthe subject of echo phenomena istreated. A difference in. the conditions aiiecting these phenomena intour-wire and two-wire circuits, and which largely determines themeasures required for their prevention, relates to the dinerence in thetime required for the propogation of the singing or echodisturbances'through their round-trip paths, on account of thediiference in the types and lengths of suchpaths. (In the case oftwo-wire circuits the practical case is assumed in which singing andecho currents circulate in the four=-wire local circuit at repeaterpoints between two adjacent line sections.)

are illustrated in this application two forms of gression of the initialsignal impulse around the singing or echo path being suflicient toinsure the completion of the switching operation by the time the echocurrents are incident on the receiver.

A similar operation can be performed at a repeater station in a two-wirecircuit by inserting a delay circuit" in each branch of the localfour-wire circuit, ahead (in the direction of transmission) of thepoints whence the relay circuits are derived.

These delay circuits are defined sufficiently by a statement of theirfunction. They must delay the transmission by the required amount and indoing so must not impress on the transmitted currents any other eiiect,although particular designs of delay circuit may, for example, be usedto give a delay which is different for the different frequencies in aband and therefore to compensate for an equal and opposite delay whichis already present in the transmissions and which is an inherentcharacteristic of transmission circuits generally. Such a delay circuitis disclosed and claimed in Patent 1,633,6l5,granted June 28, 1927. Bandpass filters having a suflicient number of sections (as many'as 100 havebeen required in known instances) may be used. There mechanical (oracoustical) delay circuits in which electrical energy is transformedinto mechanical energy and retransformed into electrical energy, thedelay occurring during the mechanical phase.

In an alternative operation of a two-wire cir cuit the two branches maynormally be in an inoperative condition and signaling currents may beused to actuate switching means to make its own branch operative. Underthis operation the delay circuits are positioned as before but their useis required for a difierent reason, that is, to prevent clipping of thefirst part of the signal transmission while that branch of the circuitis being conditioned for transmission.

The system in which the present invention has its most effectiveembodiment is a four-wire circuit of a unique type. In accordance withone practical application for which it has been suggested it may beassumed for convenience of delink and the land line.

scription that it comprises a radio link connecting Rocky Point, LongIsland and London, England and a low frequency land line connectingRocky Point and New York city. The radio link obviously does notinterpose any substantial delay to signal transmission. The land lineinterposes some delay, but on account of its relatively short length,not enough to simulate the function of the usual four-wire land circuitas above described. though of the four-wire type, approximates, so faras impedance characteristics are concerned, the type of four-wirecircuit used at local repeater circuits of a two-wire circuit. However,the presence of the relatively short land line portion, to the extentthat it may be made use ful, differentiates the circuit from bothfourwire and two-wire circuits of known type and from any combination ofthem.

In the method of operation of such a circuit as provided by thisinvention, the switching means are located at the junction of the radioThe west branch (the branch used for westward transmission) is normallyoperative and is rendered inoperative by eastward signal transmission, adelay circuit b ing inserted to insure that the switch which shortcircuits the west branch isactuated before the return current isincident on the part of that branch where the short circuit occurs. Theeast branch is normally inoperative and is made operative by theeastward transmission at the same time that the west branch is madeinoperative, and by the same relay control circuit. The

delay circuit accordingly functions both as above described and also forpreventing the clipping of the eastward signals. The westward transmission is concerned in the switching function only to the extent that it,through a similar relay control circuit, open-circuits, the other relaycontrol circuit so as to prevent current that passes through the balancecircuits connecting thefourwire with the terminal two-wire circuit fromenergizing that control circuit to reorganize the circuit as awhole fortransmission. The land line introduces sufficient delay to enable atimely performance of this function although it might not introducesufiicient delay to enable the received currents to render thetransmitting branch inoperative for transmission by closing a shortcircuit, if it were attempted to operate the circuit as in conventionalfour-wire practice, without the use of elaborate circuit means to'insurequick action. 1

A radio circuit of this type is subject tofurther difiiculties ofoperation, as compared with an all wire circuit, which makes itdesirable to maintain a breakin the roundtrip singing path as is heredone by keeping the eastward branch inoperative. This'is due to thephenomena of fading whereby the loss in the radio part of the circuit issubject to great variations. If both branches are maintained operativeat the best gain for a certain condition of the intervening space, adiminution of the transmitting loss would result in the circuit singing.I

The above and other objects and features of the invention will becomeapparent by a reading of the following detail description in connectionwith the accompanying drawing.

Fig. 1 of which illustrates a preferred embodiment of the invention, asapplied to a four-wire circuit of the type described above and 7 Figs. 2and 3 illustrate forms of mechanical Accordingly, the circuit, al

later.

Ali/I5 and AMe.

delay circuits that may be used in the system of Fig. 1.

Fig. 4 illustrates a form of an electrical delay circuit.

Referring to the Fig. 1 L1 and L2 indicate low frequency lines conected, for example, to central telephone exchanges. ln'the particularcircuit that will be assumed for convenience, L1 is connected to acentral station in New York city and L2 to a central station in London.The

circuits between these lines are adapted to transmit signals in bothdirections over individual paths,'partly by wire and partly by radio.The radio link comprises the portions between antennae AN1 and ANZ forthe east branch of the four-wire circuit and between antennae ANs and AMfor the west branch of the four-wire circuit. Antennae Arh and AN3 inthe practical embodiment assumed are located at or near Rocky Point,Long Island, and antennae ANz and AN4 are located at London, England.The circuits at the east station connecting the antennae with the lowfrequency line comprise, in the receiving portion, the receiving circuitRC1 and amplifier AM1, the corresponding circuits for the transmittingportion comprising transmitter T1 and amplifier AMz. These transmittingand receiving portions of the circuit are joined to the low frequencyline L2 by means of triple Winding transformer or hybrid coilarrangement H1 which cooperates with line L2 and with balancing networkN1 in a well-known manner to insure conjugacy between the transmittingand receiving portions. The amplifiers AM1 and Alvis may be used or notas desired. Expediency will justify their use in the usual practicalcase. The transmitted T1 and receiver RC1 embody combinations of conventional elements for respectively generating and transmitting a signalmodulated high frequency wave and for demodulating an incoming signalmodulated high frequency Wave to reproduce the signal. The antenna ANZand AN; are geographically or geometrically related to each other insuch manner that local singing at that station is prevented. This effectmay be aided by the use of different frequencies for the two directionsof transmission and the frequency selection at the antennae madepossible thereby.

The circuits at the west station between the antenna AN1 and ANs and thelow frequency line L1 comprise a low frequency transmission path ofseveral miles in length. The transmitting and receiving portions of thispath are ,.conjugately related to low frequency line L1 by flowtherethrough or it may be of the mechanical type illustrated in Figs. 2and 3 and described The criteria for the eiiicient accomplishment ofthis function will be described later. The transmission circuit forreceiving at this station may similarly comprise the amplifiers AntennaeAN1 and AN3 are so related to each other, by any of the means referredto in the description of the east station circuits, as to preventsinging at the west station. For example, one or both of these antennaemay be made directive in such a direction as to exclude reaction on orfrom the other antenna. As a practical matter it will also be foundexpedient to position these antennae amile or more apart. Amplifiers AM4and AMs may conveniently be located at their respective antennae.Amplifiers AMs and AM5 may together constitute a four-wire repeatingstation located intermediate the antenna: and the low frequency line L1.

The provision against local singing at the two stations insures that thesystem as a whole func tions as a unitary four-wire line and radiocircuit.

The west branch of the four-wire circuit is normally operative but ismade inoperative by actuation of the switch 1 by relay R1. The castbranch of the four-wire circuit is normally inoperative by reason of theshort circuit provided by switch 2 which may be actuated to opencircuitposition by relay R2. For efficient operation of such a four-wirecircuit it is desirable that only that branch be operative fortransmission which is being employed at the moment. This insures thatthe round-trip path for singing or echo currents is at all timesinterrupted. The means for actuating the switches 1 and 2 so as tosatisfy this requirement will now be described.

During transmission east, signal currents flow through amplifiers AM3and AM4 and thence through delay circuit D to the radio transmitter. Arelay control circuit 3 is derived from this branch at the input of thedelay circuit. Signal currents flowing through this control circuit areamplified and rectified by device AMR and the resultant currents flowthrough and actuate relays R1 and R2. Relay R1 actuates switch 1 tocircuit closing position and accordingly renders the west branchinoperative. Relay R2 actuates switch 2 to open circuit position andaccordingly renders the east branch operative.

The delay circuit D insures that the actuation of switch 1 will havebeen completed before echo currents transmitted to and returned from theeast station have progressed to the points of connection of the circuitcontaining switch 1. This delay circuit also insures that the switch 2will have been actuated to render the east branch operative before theinitial signal impulses reach the output of the delay circuit, so thatclipping is prevented. Since it requires less time to open switch 2 thanit does to close switch 1, the design of a delay circuit to satisfy thefirst condition would ordinarily be effective for satisfying the secondcondition. This obviously would not be true if the west branch were madeinoperative by a circuit opening operation, as would be possible by asimple rearrangement of the circuits. However it has been foundexpedient in a practical case to use the circuit closing alternativedisclosed. The amplifier rectifier AME may comprise any means foroperating on the signal currents tov obtain a wave which can efficientlyoperate relays R1 and R2. In a practical case it would comprise acombination of amplifier and rectifier elements, preferably of thevacuum tube type. An example of an arrangement well adapted for thepurpose is disclosed in Patent 1,588,186, granted June 8, 1926. Theswitches l and 2 are biased, by means not shown, so as to returnrespectively to their open and closed positions when not actuated by therelays R1 and R2.

During west transmission the detected signal currents will find anuninterrupted path to the low frequency line L1. Since the east branchis normally inoperative, the currents which pass through the hybrid coilH2 cannot result in singingor echo phenomena in the four-wire circuit asa whole. However, in order to insure that such currents cannot entercontrol ,circuit 3, which would reorganize the circuits for eastwardtransmission and accordingly produce a periodical vibration of theswitches l and 2 with an attendantinterruption of the received signalsand a tendency to singing and echo phenomena, a portion of the receivedsignal currents interrupts this relay control circuit by opening theswitch 4. This is accomplished by relay R3 in relay control circuit 5.Amplifier rectifier AMRl in this circuit has the same function as thatof AMR in circuit 3. The delay in transmission over the land line fromthe output of amplifier AMG to the output of amplifier AM4 insures thatswitch 4 will be opened before these currents reach the output ofamplifier AMl.

This delay would not ordinarily be sufiicient if it were attempted toutilize the received signal currents to actuate a circuit closing meansto render the east branch inoperative or at least it would not besufficient without the employment of more elaborate means than would berequired for circuit opening. Therefore, if it were attempted tooperatethis circuit, to prevent singing and echo phenomena, in theconventional manner of 22-type repeaters in a two-wire circuit, it wouldbe necessary to insert a delay circuit in the west branch ahead of thepoints from which the relay control circuit 5 is derived. Therefore inaccordance with this invention a situation, which in accord ance withthe prior art would have to be met by the use of two delay circuits, isequally well met by the use of a single delay circuit, a'novel fea--ture incident to the use of the single delay circuit being a biased typefour-wire circuit, that is, a four-wire circuit in which one portion isnormally operative and the other portion is normally inoperative. Thebasic principle of the invention is applicable also to a circuit havingno inherent delay characteristics, as in a 22 type repeater, by anarrangement substantially as described but in which a delay circuit ofrelatively few sections replaces the low frequency Such an arrangementwould be an land line. improvement over the conventional two delaycircuit arrangement in that the delay accomplished by the second delaycircuit would need to be very small, and the cost of' the delay circuitcorrespondingly very low, in comparison with the other delay circuit.

The general nature of delay circuits has been pointed out above.Specific types of such circuit adapted to be used in the system of Fig.1 will 1 tiesforce, displacement, velocity, mass, stiffnessCorresponding to each electrical and friction. system there may beconceived a mechanical system constituted by mechanical elements havinganalogous properties and combined to produce analogous results.

In electrical phenomena a time relay is a phase the second inductanceelement.

change of relatively higher order than occurs in simple reactivenetworks. Therefore, itis reasonable to expect an electrical delaycircuit to consist of a reiterative series of such simple networks so asto repeat and continue the phase shifting effect of each unit throughsuccessive units to eventually obtain any desired delay. Obviously, inorder to produce a periodic repetition of the effect of a single unitboth magnetic and electrostatic storage elements, that is, bothinductance and capacity, are required. Such an expedient carried out ina systematic manner to give a uniform sheet for a band of frequenciesresults in a structure that may be identical with conventionalattenuation type filter structure,

except that it has, inthe practical case, many more sections. In fact,the delay principle is inherent in the operation of such filters and isnecessary to the development of their theory of operation. 7

Fig. 4 illustrates a simple typeoifilter, a lowpass filter, which isadaptable to use as a delay circuit if provided with a sufficient numberor" sections. The-theory of the operation of this and other types offilter is developed in U. S. patent to Campbell 1,227,113, granted May22,1917. The physical principle involved is explained in an article byCampbell in the Bell System Technical Journal for November 1922 entitledPhysical theory of the electric wave filter, wherein it is shown onpages 10 et seq. that transmission through the filter is attended by aprogressive change of phase from section to section for each frequencycomponent. If a certain amount of magnetic energy is stored in the firstinductance element, it discharges into the first shunt condenser,building up a counter E. M. F. therein. A time which is determined bythe time constant of one section, comprising the combination of oneinductance and one capacity, is required for this operation. When thecondenser has received its maximum charge, it discharges into This cycleis repeated for each section, the total delay accordingly beingdetermined by a choice of the number of sections.

Fig. 2 illustrates a type of mechanical delay circuit which is theanalogue of the electrical circuit of Fig. i, the mass of elements M andthe stiffness of elements Sbeing analogous respectively to theinductance of the coils and the capacity of the condensers of Fig. i.The design data pertaining to electrical delay circuits as pointed outin the Campbell references above mentionedare equally applicable to themechanical delay circuit of Fig. 2, having regard for the differences inunits and conversionfactors. For ideal conditions simulating thoseexisting in..a resistanceless electrical filter circuit made up oflumped inductances and capacities each of which is completely individualin its function, the massesmust be assumed to have zero dimensions, thestiffness elements, for example, springs, must be assumed to have noweight, pendant means P must be assumed to he perfect- 1y flexible andthere must be no frictionanywhere in the system. Conversion fromelectrical to mechanical energy and back to electrical energy isaccomplished respectively by the electromagnetic receiver device R andthe microphone transmitter device T, which function as in accordancewith similarly denominated devices in systems of telephone transmissionThe first mass element at the left is impulsed by the receiverdiaphragm. In a manner entirely analogous to the operation of the endsection of the network of Fig. 4, the energy is absorbed in actuatingthe mass into movement. After an; interval determined by the timeconstant of the first section comprising a mass and a stiffness element,this energy will have been stored in the mass and then expended instoring up an equivalent amount of energy in the first stiffnesselement. This stiffness element, the first spring, in turn dischargesthis energy into the second mass element, and soon throughout thetransmission path. It is substantially immaterial so far as the physicalprinciple is concerned whether the mecahnical energy transfers occur ina single plane including the several pendant means ,P or in planestransverse thereto. The number of sections, in a practical case, wouldbe many times as great as that shown.

Fig. 3 illustrates an acoustical delay circuit. The receiver andtransmitter elements are the same as in Fig. 2. Intermediatetransmission is by means of acoustic waves directed and regulated by thesound conduit 0 as in accordance with conventional practice in theconstruction of the horn elements in acoustic transmitters andreceivers. The delay in this type of transmitting medium is inherent inthe material of the medium, although if the elem nts of this medium werereduced to molecular dii -ensions it would be obvious that theconditions inherent in the op eration of the arrangement or Fig. 2 areclosely paralleled. In common in the two systems, transmission effectedby wave motion, the velocity of propagation being determined by the mass(or density) and the stiffness or elasticity) characteristics of themedium. In the arrangement of fig. 3 the desired delay is regulatedsolely by vary ng the length of the path since the characteri tics ofthe medium are fixed. In the arrangement of Fig. this delay iscorrespondingly regulated both by the length of the path and thecharacter of the medium, since the transmission characteristics of themedium are subject to adjustment.

, Having described the nature of the invention and its mode of operationthe features believed to be novel are set forth in the appended claims.

What is claimed is:

1. A two-way signaling system comprising two paths adapted fortransmission in opposite directions between two stations, meansconnecting said paths in one 'gy flow relation at each station, andautomatically operative means including a single delay circuit, insertedin the transmission path corresponding to one direction, to theexclusion of a similar delay circuit in the other path or elsewhere, forpreventing round-trip transmission of echo and singing currents.

2. A two-way signaling system comprising two paths adapted fortransmission in opposite directions between two stations, a circuitconnected in energy flow relation with each of said paths at eachstation, means in one path to render it normally inoperative fortransmission, means in the other path for rendering it normallyoperative for transmission, means whereby energy impressed on the firstmentioned path renders the normally inoperative path operative fortransmission and the normally operative path inoperative fortransmission, and a single delay circuit, which is inserted the firstmentioned path, to the ex clusion or a similar delay circuit in theother path or elsewhere, adapted to delay the roundtrip transmission ofecho currents until said operative path has been made inoperative.

3. A two-way signaling system comprising two paths adapted fortransmission in opposite directions between two stations, meansconnecting said paths in energy flow relation at each station, means inone path to render it normally inoperative for transmission, means inthe other path for rendering it normally operative for transmission,means whereby energy impressed on the first mentioned path renders thenormally inoperative path operative for transmission and the normallyoperative path inoperative for transmission, a delay circuit adapted todelay the round-trip transmission of echo and singing currents untilsaid operative path has been made inoperative, and means whereby energyimpressed on the second mentioned path renders the last mentioned meansinoperative, whereby energy in the first mentioned path derived from thesecond mentioned path cannot cause an undesirable reorganization ofcircuits, said last two mentioned means being separated by a sufiicientlength of transmission circuit including portions of both of said pathsto delay the transmission of said derived energy, without the use ofother delay means, until the last mentioned means has operated.

4. A four-wire radio signaling system comprising means normallyrendering one branch operative for transmission, means normallyrendering the other branch inoperative for transmission, signalresponsive means for substantially simultaneously rendering saidnormally operative path inoperative and said normally inoperative pathoperative for transmission and a single delay circuit, said delaycircuit being the only one used in the entire four-wire system,posititined in the normally inoperative branch for preventing round triptransmission of echo currents over the loop constituted by said fourwirecircuit during the operation of said signal operated means.

5. The system of claim 4 in which the signal responsive means isresponsive to signal transmission over the normally inoperative path.

6. In combination a two-wire circuit, a second two-wire circuit, afour-wire circuit between said circuits comprising a wire portion and aradio link, biased means normally rendering one branch of said four-wirecircuit operative, biased means for normally rendering the other branchinoperative, a circuit in shunt to the normally inoperative branch,said. circuit containing relay means for actuating each of said biasedmeans to substantially simultaneously render the said normallyinoperative circuit operative and said normally operative circuitinoperative for transmission, the combination as a whole including onlyone delay circuit, said delay circuit being positioned in said normallyinoperative branch between said shunt circuit and the biased means, saiddelay circuit being adapted to delay the transmission to each of saidbiased means until they have respectively been actuated by said relaymeans.

7. The system of claim 6 in which the wire portion is a low frequencytransmission line and the radio link comprises in each branch means fortransforming the impressed low frequency currents to a high frequencywave modulated by the low frequency currents, and for retransforming itback to the low frequency currents at the other end of the radio link.

8. In combination a two-wire low frequency circuit, a second two-wirelow frequency circuit, a four-wire circuit between said circuitscomprising in one branch in series in the given order, a low frequencytransmission line, a delay circuit, a radio transmitter and a radioreceiver, and in the other branch, symmetrically with the elements ofthe first branch, a second low frequency transmission line, a radioreceiver, and a radio transmitter; a shunt circuit containing a switchnormally biased to closed position across the output of the delaycircuit, said delay circuit being in the normally inoperative path andalso being the only one used in the entire four-wire circuit, a shuntcircuit containing a switch normally biased to open position across theinput end of the second low frequency transmission line, a relay controlcircuit in shunt to the first low frequency transmission linesubstantially at its output end, a relay control circuit in shunt to thesecond low frequency transmission line substantially at its input end,the first mentioned relay control circuit comprising relay means foractuating both the said switches to their respectively alternatepositions and a switch normally biased to closed position, and thesecond mentioned relay control circuit containing relay means foractuating the switch in the other relay control circuit to openposition.

9. In combination a two-wire circuit, a second two-wire circuit, afour-wire circuit between said circuits comprising a wire portion and aradio link, means normally rendering one branch of the four-wire circuitoperative for transmission and the other branch normally inoperative fortransmission, signal responsive control means for substantiallysimultaneously rendering said normally operative branch inoperative andthe normally inoperative branch operative for transmission, a delaycircuit in the normally inoperative branch .posterior to the connectionof said control means thereto, said delay circuit being the only oneused in the entire four-wire circuit, and a second control means forcontrolling the operation of the first control means, said secondcontrol means utilizing the delay inherent in the propagation of signalsalong said wire portion of the four-wire circuit.

ROBERT C. MATHES.

